Origin and distribution of natural gas and oil in the Baiyun Depression, Pearl River Mouth Basin, South China Sea

The Baiyun Depression is the main hydrocarbon-rich province in the deep-water area of the Pearl River Mouth Basin, South China Sea. Several petroleum samples, well data and 3D-seismic data from the Baiyun Depression were collected, the geochemical features and origins were determined using various methods, and finally, their distribution pattern was analysed. The results are as follows: (1) the natural gases are the thermogenic gases of organic matter belonging to terrestrial organic gas. They are mainly kerogen-cracking gases with high maturities, and their kerogen is the mixed sapropelic-humic type. The carbon isotope of the oil indicates that its source rock may be a continental deposit. Most of the oil in the Baiyun Depression has suffered from gas washing, which substantially affected its light hydrocarbon content. The biomarker features of the oil contain high Pr/Ph (pristane/phytane), high oleanane and medium bicadinane, reflecting that its source rock contains a large amount of terrestrial organic matter. The steranes indicate that the oil is mature and is derived from both higher plant and algae. It can be concluded that natural gas and oil in the Baiyun Depression mainly originated from source rocks of Enping Formation. (2) The distribution pattern of natural gas and oil is mainly controlled by the characteristics of the hydrocarbon source kitchens and the hydrocarbon migration pathways. These results can provide some information for future petroleum exploration in the Baiyun Depression, as well as in other petroliferous basins with similar geological settings.